If you want to  join in Indian army at a higher post then you should be prepare for N.D.A (National Defense Academy) and C.D.S(Combined Defense Service) like exam. These are all exam is conducted by U.P.S.C (Union Public Service Commission). These exam is conducted in every year twice.there are many more option for you like soldier clerk, technical, J.C.O rank ,havildaar educational and many more etc. But if you are going with higher rank then you can apply only for N.D.A and C.D.S.

If you want to apply after 12th then you can apply for NDA. You can check the eligibility with given below link.

NDA eligibility

If you want to apply after graduation then you can apply CDS. You can check the eligibility.

CDS Eligibility